1. Field of the Invention
The present invention relates to a method of producing a friction material, particularly a clutch facing, used in vehicles such as automobiles. More particularly, the present invention is directed to a clutch facing prepared without using asbestos fibers.
2. Description of the Prior Art
In the past friction materials which have been utilized in the preparation of clutch facings have been prepared by various processes all of which use asbestos fibers as a base material. However, because asbestos fibers are believed to be harmful to human health, the production of the friction materials which contain asbestos fibers is accompanied by such concerns as the state of the working environment in which the friction materials are prepared as it relates to workers' health.
In view of this problem, active studies have recently been started in the friction material industry in order to develop a friction material which does not contain asbestos fibers. As a result, several kinds of friction materials have been proposed. One such proposed friction material is one which is based upon steel fibers as the base material. However, because steel generally has a large specific gravity, its use results in an increase in weight of the resulting friction material, and consequently the required moment of inertia is not obtainable. Friction materials based upon steel fiber have the further disadvantage that they are apt to rust and as a result stick to the mating material thereby causing failure in the function of the clutch, as well as other problems. Organic natural fibers and organic synthetic fibers generally available on the market which are free of these drawbacks have also been studied as base materials for friction materials. However, these fibers are all inferior from the viewpoint of being impervious to heat and from the viewpoint of strength, so that a good friction material, which is not based upon asbestos, has not been available.
Other fiber materials have also been proposed which have superior heat resistance such as glass fibers, phenol fibers and carbon fibers. In order to prepare friction materials from such fibers, the fibers are impregnated with binders and additives and the resulting impregnated fibers are subjected to a pressure/heat forming process. However, the surfaces of these fibers are even surfaces with no roughness so that their adhesion to the binder is insufficient and the desired strength of a friction material product is not obtained. In order to solve this problem it has been proposed to make the diameter of the fibers smaller, but this results in an increase in cost, thus making their use difficult. A need, therefore, continues to exist for a friction material based upon a fibrous material other than asbestos of improved characteristics.